Abstract

Abstract
Idiomorphic, lath-like lM- and hexagonally shaped 2M1-illites from the clay gouge of the hydrothermal fluorite vein deposit Käfersteige (Northern Black Forest, Germany) were investigated by energy dispersive X-ray spectroscopy (EDX) and convergent beam electron diffraction (CBED) methods in the transmission electron microscope (TEM). All observations were made with a 50 nm focussed beam and CBED experiments were performed parallel to c* of the crystallites. The 1M- and 2M1-particles produced different spot patterns during diffraction, allowing reliable identification. According to their grain size distribution curve and idiomorphic habit, both mineral types should have crystallized cogenetically. Additionally, EDX analyses on single crystals show variable chemical composition between crystallites of the same modification; also the fields of 1M- and 2M1-illites are sharply discriminated. Therefore 1M- and 2M1-illites may no longer be considered polytypes; instead, they are different mineral phases.